(1) Field of the Invention
The present invention relates to a switching power supply device having an overvoltage protecting function.
(2) Description of the Related Art
In conventional switching power supply devices that include a power transformer, it is common to detect an output voltage on the secondary side using a control integrated circuit (IC) or the like provided on the secondary side and to provide feedback of information on the detected secondary-side output voltage to the primary side using a photocoupler.
However, the expensive secondary-side control IC and photocoupler constitute a large part of the cost of the switching power supply device and inhibit miniaturization of the switching power supply device.
In view of such drawbacks, switching power supply devices have been proposed which detect the secondary-side output voltage and control the secondary-side output voltage to be constant on the primary side without using the secondary-side control IC and the photocoupler.
In addition, as described in Japanese Patent Application No. 3973652 (hereinafter referred to as Patent Reference 1), there has been a proposal of a technique for controlling a secondary-side output current to be constant on the primary side without using the photocoupler and so on.
Patent Reference 1 proposes a switching power supply device which performs the following controls in combination by making use of the property of the voltage induced in an auxiliary winding of a transformer to fluctuate according to the secondary-side output voltage: (i) constant voltage control implemented through feedback control using an auxiliary winding, by which the secondary-side output voltage is controlled to be constant and (ii) constant current control implemented through control performed for fixing the on-duty of a secondary current, by which the secondary-side output current is detected from the pulse width of the auxiliary winding voltage waveform and the secondary-side output current is controlled to be constant.
Since such a switching power supply device handles high currents and voltages, various safety standards are in place. Furthermore, many of the power supply manufacturers begin mass production on the condition that the safety is confirmed through their own unique abnormality tests. One of such abnormality tests is a terminal opening test. The terminal opening test allows suppression of a rise in the output voltage even when a control IC, a transformer, or another component is missing or even when poor adhesion occurs. The terminal opening test thus enables a safety check.